1. Field of the Invention
The present invention relates to a level detection apparatus, for example an auto-focus technique of a pattern inspection apparatus which inspects a pattern defect of an object serving as a sample used in semiconductor manufacturing. The present invention also relates to an auto-focus technique of a pattern inspection apparatus to inspect a defect of a lithography mask used when a semiconductor device or a liquid crystal display (LCD) is manufactured.
2. Related Art
In order to irradiate light on a surface of an object to measure a level of a target object surface from the reflected light from the surface, the following technique is used as a conventional method. Light from a light source is converged to form a beam spot on the target object. A circular pin hole which focuses the reflected light and splits a beam of light in front of a focal point to regulate one of the beams of light in front of the focal point is installed, and a circular pin hole to regulate the other of the beams of light in back of the focal point is installed. Amounts of lights passing through the circular pin holes are detected by separate sensors, respectively. A level of the target object is detected on the basis of a ratio of the amounts of light (for example, see Japanese Patent Application, Publication No. JP-A-05-297262).
However, this method has the following problems. First, an amount of light is insufficient in an optical system such as a Koehler illumination system in which a point source cannot be easily formed.
Secondly, when an image obtained from a target object crosses a boundary of different reflection distributions of the target object, an error easily occurs due to the reflectance distribution of the object. The problem can be avoided when the optical positions of the circular pin holes installed in front of or in back of the focal point completely coincide with each other. However, when the positions are different from each other in installation of the circular pin holes, a large amount of light may enter into one of the circular pin holes, and only a relatively small amount of light may enter the other pin hole. In this case, since light amounts cannot be accurately detected, a detected level position of the target object has an error. However, it is difficult to cause the optical positions of the two circuit pin holes to completely coincide with each other. When the circular pin holes are used, the detection is disadvantageously influenced by the reflectance distribution.
Thirdly, when diffracted light caused by a pattern of a target object is generated, an error is easily generated by the influence of the diffracted light. This can be avoided when the optical positions of circular pin holes installed in front of and in back of a focal point. However, when the positions are different from each other in the installation of the pin holes, a large amount of diffracted light may enter into one of the circular pin holes, and only a relatively small amount of light may enter into the other circular pin hole. In this case, since the amounts of light cannot be accurately detected, a detected level position of the target object has an error. However, as described above, it is difficult to cause the optical positions of the two circuit pin holes to completely coincide with each other. When the circular pin holes are used, the detection is disadvantageously influenced by the diffracted light generated by a periodic pattern of the target object.